1. Field of the Invention
This invention relates generally to optical data storage systems and more particularly to a disk storage system and a method for reducing the servo crosstalk between the focus and tracking error signals.
2. Description of the Prior Art
Optical data storage systems provide a means for storing great quantities of data on a disk storage medium. The data is accessed by focusing a laser beam onto the disk and then detecting the information encoded in the reflected light beam. Various kinds of optical systems are known including read only memory (ROM), write once read many (WORM) and erasable. The erasable systems include magneto-optic (MO) and phase change recording media.
The data is typically stored on the optical disk in concentric or spiral data tracks. The laser must maintain focus on the disk while at the same time following the data tracks on the rotating disk. The focus is maintained by moving an objective lens relative to the disk with a focus actuator. The tracking position is adjusted by moving the position of the optical head in a radial direction with a linear actuator. The optical head typically contains the focus lens, focus actuator and beam bender mirror. Various focus and tracking servo schemes are known in the art.
One of the most widely used track servo schemes uses tracking grooves. The data track regions are also referred to as lands. The tracking grooves are molded into the disk on either side of each data track. Grooves may be V-shaped or trapezoidal shaped. The focussed beam has a Gaussian diameter comparable to the land width and is focussed to the groove-land structure of the disk. Diffraction produces different intensity distributions between the zero and plus or minus first diffraction orders depending upon the radial position of the beam spot with respect to the grooves. Generally more intensity is diffracted into the first orders as the beam moves away from the land section and toward the groove section. These light intensity distributions are imaged onto a servo optical detector and used to generate a tracking error signal (TES) which in turn controls the linear actuator to move the optical head in order to keep the beam on track. The same optical detector is commonly also used to generate the focus error signal (FES) as well as the TES. The FES is used to control the focus actuator to maintain the light beam in focus on the disk.
A problem develops when the optical head is moved radially across the disk to seek different tracks. The diffraction of light at the grooves causes wavefront aberrations in the reflected light which results in distortions of the FES. This can cause the light beam to go completely out of focus during the seek operation and may result in disk drive losing track of the position of the optical head, i.e. reduced seek reliability. This phenomenon is referred to as optical servo crosstalk of the TES into the FES and is discussed in the article by I. Prikryl, "Optical Servo Crosstalk," Optical Society of America, Technical Digest Series, Optical Data Storage, Vol. 1, January 1989.
Prior art inventions have sought to solve this problem. These include Japanese patent applications JP 01-89043, published Apr. 3, 1989; JP 63-157326, published Jun. 30, 1988; and JP 62-146439, published Jun. 30, 1987. Each of these systems require rather complex circuitry and/or optical components to obtain a slight reduction in servo crosstalk. What is needed is a simple and more practical solution.